In a conventional reflection type bar, printing ink containing carbon black is generally used. A bar code is printed on a sheet of paper with the printing ink, and a bar code reader optically detects the difference between the reflectivity of a bar code-printed portion of the paper and that of a bar code-unprinted portion thereof so as to read the code information which the bar code has.
The reflection type bar has, however, a disadvantage of causing errors in reading the information of the bar code in that the appearance of the commodity is damaged because the bar code is printed on the surface of a commodity or the like or the above-described reflectivity difference becomes small when the bar code-printed surface of the commodity is soiled.
In order to overcome the disadvantage, there are proposed various methods of printing a latent image mark containing fluorescent substances and projecting infrared rays to the latent image mark so as to optically detect light emitted by the latent image mark.
FIG. 12 is a view for describing this kind of conventional optical reading apparatus. A latent image mark 100 such as a bar code is printed on a mark carrier 101, for example, a commodity, a component or the like. The latent image mark 100 contains fluorescent fine powder. A projecting member 103 projects light for exciting the fluorescent substances and as a result, fluorescence is emitted by the latent image mark 100 and received by a light-receiving member 104, so that the code information of the latent image mark 100 is read optically.
When the above-described conventional optical reading apparatus is used as a handy optical reading apparatus, it is incapable of reading the code information of the latent image mark 100 correctly, thus causing erroneous detection and being unreliable in its performance.
Having made various researches on the causes, the present inventors found that the angle at which the optical axis of the projecting member 103 of the optical reading apparatus intersects with that of the light-receiving member 104 thereof and the state of the surface of the mark carrier 101 on which the latent image mark 100 has been printed relate to reading accuracy greatly.
That is, assume that the optical reading apparatus is mechanically fixed to a predetermined position; the latent image mark 100 is printed on the mark carrier 101 such as a sheet of paper having a flat surface; and the mark carrier 101 is guided by a guide member mounted on the optical reading apparatus. If the distance between the projecting member 103 and the latent image mark 100 and the distance between the light-receiving member 104 and the latent image mark 100 are always constant, the code information of the latent image mark 100 can be read comparatively correctly, even though the intersection angle .theta. formed between the optical axis of the projecting member 103 and that of the light-receiving member 104 is as great as 45.degree.-60.degree. as shown in FIG. 12.
In the case of a manually operated handy optical reading apparatus, the reading position of the light-receiving member 104 becomes nonuniform in relation to the latent image mark 100 or the detection aperture surface of the optical reading apparatus becomes oblique or moves in relation to the surface of the latent image mark 100. Further, when the surface state of the mark carrier 101 on which the latent image mark 100 has been printed is irregularly convex or concave as in the case of a confectionery bag, or curved or stepped as in the case of a component, the distance between the projecting member 103 and the latent image mark 100 and the distance between the light-receiving member 104 and the latent image mark 100 become nonuniform, respectively.
It has been found that in reading the code information of the latent image mark 100 in such a condition, if the intersection angle .theta. formed between the optical axis of the projecting member 103 and that of the light-receiving member 104 is as great as 45.degree.-60.degree., the code information of the latent image mark 100 cannot be correctly read, thus causing erroneous detection of the code information of the latent image mark 100.